Last year we found that regulatory T cells infiltrated into the liver of female Balb/cJ mice following halothane treatment and inhibited both humoral and cellular immune reactions against trifluoroacetylated proteins formed in vivo following halothane treatment. This year we report that newly discovered myeloid-derived suppressor cells also infiltrated into the liver of female Balb/cJ mice following halothane treatment. These cells have be shown recently to inhibit effector T cell activity in tumors, infections, and in autoimmune diseases. Conclusion: These findings suggest that both regulatory T cells and myeloid-derived suppressor cells might have a role in preventing drug-protein adducts from causing liver injury mediated by the adaptive immune system and that deficiencies in the activities of these regulatory cells might be susceptibility factors in DILD.